The present invention relates to high-density acicular hematite particles, a non-magnetic undercoat layer containing the high-density acicular hematite particles and a magnetic recording medium having the non-magnetic undercoat layer. More particularly, the present invention relates to high-density acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium using magnetic particles containing iron as a main ingredient, which have an excellent dispersibility in binder resin, a pH value of not less than 8, a less content of soluble sodium salts and soluble sulfates, and a high surface conductivity; a non-magnetic undercoat layer containing the high-density acicular hematite particles and suitably used for a magnetic recording medium using magnetic particles containing iron as a main ingredient; and a magnetic recording medium having the non-magnetic undercoat layer.
With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Various attempts have been made at both enhancing the properties of magnetic particles and reducing the thickness of a magnetic recording layer in order to improve these properties of a magnetic recording medium.
The enhancement of the properties of magnetic particles will first be described.
Magnetic particles are required to have, in order to satisfy the above-described demands on a magnetic recording medium, properties such as a high coercive force and a large saturation magnetization.
As magnetic particles suitable for high-output and high-density recording, acicular magnetic particles containing iron as a main ingredient which are obtained by heat-treating acicular goethite particles or acicular hematite particles in a reducing gas are widely known.
Acicular magnetic particles containing iron as a main ingredient have a high coercive force and a large saturation magnetization, since the acicular magnetic particles containing iron as a main ingredient used for a magnetic recording medium are very fine particles having a particle size of not more than 1 .mu.m, particularly, 0.01 to 0.3 .mu.m. Therefore, such particles easily corrode, and the magnetic properties thereof are deteriorated, especially, the saturation magnetization and the coercive force are reduced.
In order to maintain the characteristics of a magnetic recording medium which uses magnetic particles containing iron as a main ingredient as the magnetic particles, over a long period, it is strongly demanded to suppress the corrosion of the acicular magnetic particles containing iron as a main ingredient as much as possible.
A reduction in the thickness of a magnetic recording layer will now be described.
Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
With respect to short wavelength signals, a reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, an S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co., Ltd. (1982), " . . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness . . . ".
Development of a reduction in the thickness of a magnetic recording layer has caused some problems.
Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the substrate must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology-Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as "Materials for Synthetic Technology" (1987), published by the Publishing Department of Technology Information Center, " . . . the surface roughness of a hardened magnetic layer depends on the surface roughness of the substrate (back surface roughness) so largely as to be approximately proportional, . . . , since the magnetic layer is formed on the substrate, the more smooth the surface of the substrate is, the more uniform and larger head output is obtained, and the more the S/N ratio is improved."
Secondly, there has been caused a problem in the strength of a non-magnetic substrate such as a base film with a tendency of the reduction in the thickness of the non-magnetic substrate in response to the demand for a thinner magnetic layer. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, " . . . Higher recording density is a large problem assigned to the present magnetic tape. This is important in order to shorten the length of the tape and to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a substrate. . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded. . . . "
The end portion of a magnetic recording medium such as a magnetic tape, especially, a video tape is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large by a video deck. If the light transmittance of the whole part of a magnetic recording layer is made large by the thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present video tapes.
However, addition of a large amount of non-magnetic particles such as carbon black impairs not only the enhancement of the recording density but also the development of a thinner recording layer. Therefore in order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce, as much as possible, the quantity of non-magnetic particles such as carbon black which are added to a magnetic recording layer.
It is also strongly demanded that the light transmittance of a magnetic recording layer should be small even if the carbon black or the like which is added to the magnetic recording layer is reduced to a small amount. From this point of view, improvements in a substrate are now in strong demand.
Further, in order to reduce not only the above-mentioned optical transmittance but also surface resistivity of the magnetic recording medium, carbon black has been conventionally added to a magnetic recording layer thereof.
The use of carbon black in the magnetic recording medium is described in more detail below.
In the case where the magnetic recording medium has a high surface resistivity, the electrostatically charged amount on the magnetic recording medium is increased, so that cutting wastes of magnetic recording media or dusts are attached to the surface of magnetic recording medium upon production or use of the magnetic recording medium, thereby increasing occurrence of drop-out.
Consequently, in order to lower the surface resistivity of the magnetic recording medium to about 10.sup.8 .OMEGA.cm, a conductive compound such as carbon black has been generally added to a magnetic recording layer thereof in an amount of not less than about 5 parts by weight based on 100 parts by weight of magnetic particles used therein.
However, such an increase in amount of non-magnetic substance such as carbon black in the magnetic recording layer tends to cause the deterioration in signal recording property and inhibit the reduction in thickness of the magnetic recording layer.
Various efforts have been made to improve a base film for a magnetic recording layer with a demand for a thinner magnetic recording layer and a thinner non-magnetic substrate. A magnetic recording medium having at least one undercoat layer (hereinunder referred to "non-magnetic undercoat layer") comprising a binder resin and non-magnetic particles containing iron as a main ingredient such as hematite particles which are dispersed therein, on a non-magnetic substrate such as a base film has been proposed and put to practical use (Japanese Patent Publication (KOKOKU) No. 6-93297 (1994), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338 (1987), 63-187418 (1988), 4-167225 (1992), 4-325915 (1992), 5-73882 (1993), 5-182177 (1993), 5-347017 (1993), 6-60362 (1994), 9-35245 (1997), etc.)
Further, various attempts for reducing the content of carbon black in the magnetic recording layer and lowering the surface resistivity of the magnetic recording medium as low as possible, have been conducted. For example, it is known that the surfaces of non-magnetic particles dispersed in the above-mentioned non-magnetic undercoat layer are coated with a tin compound or an antimony compound (Japanese Patent Nos. 2566088 and 2566089, Japanese Patent Publication (KOKOKU) No. 5-33446(1993), Japanese Patent Applications Laid-open (KOKAI) Nos. 6-60360(1994), 7-176030(1995), 8-50718(1996), 8-203063(1996), 8-255334, 9-27116(1997) or the like).
For example, Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993) discloses a magnetic recording medium comprising: a non-magnetic substrate; a non-magnetic undercoat layer formed on the non-magnetic substrate and produced by dispersing inorganic particles in a binder resin; and a magnetic layer formed on the non-magnetic undercoat layer and produced by dispersing ferromagnetic particles in a binder resin while the non-magnetic undercoat layer is wet; wherein the magnetic layer has a thickness of not more than 1.0 .mu.m in a dried state, the non-magnetic undercoat layer contains non-magnetic inorganic particles with surface layers coated with an inorganic oxide, the inorganic oxide coating the surfaces of the non-magnetic inorganic particles contained in the non-magnetic undercoat layer is at least one selected from the group consisting of Al.sub.2 O.sub.3, SiO.sub.2 and ZrO.sub.2, and the amount of the inorganic oxide coating the non-magnetic inorganic particles is 1 to 21 wt % in the case of Al.sub.2 O.sub.3, 0.04 to 20 wt % in the case of SiO.sub.2, and 0.05 to 15 wt % in the case of ZrO.sub.2, base on the total weigh of the magnetic inorganic particles.
In Japanese Patent No. 2566088, there is described a magnetic recording medium comprising a non-magnetic substrate, a non-magnetic undercoat layer formed on the non-magnetic substrate, comprising a binder resin and non-magnetic inorganic particles dispersed in the binder resin and coated with at least one oxide selected from the group consisting of Al.sub.2 O.sub.3, SiO.sub.2, ZrO.sub.2, Sb.sub.2 O.sub.3 and ZnO, and a magnetic uppercoat layer formed on the non-magnetic undercoat layer, comprising a binder resin and ferromagnetic particles dispersed in the binder resin, wherein the magnetic uppercoat layer has a dry thickness of not more than 1.0 .mu.m; the non-magnetic undercoat layer has a dry thickness of 0.5 to 10 .mu.m; and the ferromagnetic particles have a major axial diameter of not more than 0.3 .mu.m.
At present, there has been more demanded non-magnetic particles for non-magnetic undercoat layer of a magnetic recording medium, which are capable of furnishing a non-magnetic undercoat layer having excellent surface smoothness and mechanical strength by dispersing the non-magnetic particles in a binder resin; which are capable of furnishing a magnetic recording layer having a surface smoothness and a thin and uniform thickness when the magnetic recording layer is formed on the non-magnetic undercoat layer; which are capable of furnishing a magnetic recording medium having a low transmittance and a low surface resistivity; and which are capable of preventing the corrosion of magnetic particles containing iron as a main ingredient, which are dispersed in the magnetic recording layer. However, such non-magnetic particles have not been furnished yet.
That is, it has been reported that the above-mentioned conventional magnetic recording medium using hematite particles as non-magnetic particles for non-magnetic undercoat layer thereof, are improved in surface smoothness and mechanical strength of the non-magnetic undercoat layer; is capable of forming a magnetic recording layer having a surface smoothness, and a thin and uniform thickness upon the formation of the magnetic recording layer; and exhibit a low transmittance. However, these properties reported are still unsatisfactory. Especially, as described in Comparative Examples hereinafter, the surface resistivity of these conventional magnetic recording medium is as high as 10.sup.9 to 10.sup.11 .OMEGA.cm.
On the other hand, in the case of the magnetic recording medium having the non-magnetic undercoat layer containing non-magnetic particles coated with a tin compound or an antimony compound and dispersed in a binder resin, the non-magnetic undercoat layer is deteriorated in surface smoothness and mechanical strength, though the surface resistivity thereof is low. Accordingly, the magnetic recording layer formed on such a non-magnetic undercoat layer necessarily has a rough surface and an uneven thickness, and exhibit an unsatisfactory transmittance.
Further, there has also been pointed out such a problem that the magnetic particles containing iron as a main ingredient, which are dispersed in the magnetic recording layer, undergo server corrosion after the production of the magnetic recording medium, thereby causing the considerable deterioration in magnetic properties thereof.
As a result of the present inventors' earnest studies for solving the above-mentioned problems, it has been found that by coating at least a part of surfaces of specific acicular hematite particles with an oxide of tin or an oxide of tin and antimony, and controlling the pH value to not less than 8 and contents of soluble sodium salts and soluble sulfates to a certain range, the obtained high-density acicular hematite particles exhibit a low surface resistivity and an excellent dispersibility in a vehicle. The present invention has been attained on the basis of this finding.